xen: set Sys.os_type to "Unix" instead of "xen"

The stdlib Filename module calls 'assert false' if it doesn't
recognise the os_type. Might as well be Unix.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #78 from djs55/rename-xen-to-Unix

xen: set Sys.os_type to "Unix" instead of "xen"
bump to 1.0.0
Merge pull request #79 from avsm/master

bump to 1.0.0
Xen: quieten warnings for missing prototypes so real issue can be seen.
Xen: remove dead netif code (now in mirage-net-xen)
Suppress another dietlibc linker warning for vprintf in Xen

Closes mirage/mirage#141
Merge pull request #80 from avsm/master

fixes for 1.0: quieten warnings and cleaner output